Bug#808579: apt: Very slow rred phase during update

[Stefan Fritsch <sf@sfritsch.de>]

Package: apt
Version: 1.1.5
Severity: normal

Since some time, the rred phase during aptitude/apt-get update is very
slow and produces very high system cpu-load.

Today I managed to get an strace of the relevant process (the one
matched by "pgrep -f rred", I don't know the exact name right now). It
does stuff like this:

2572  read(3, "P", 1)                   = 1
2572  read(3, "a", 1)                   = 1
2572  read(3, "c", 1)                   = 1
2572  read(3, "k", 1)                   = 1
2572  read(3, "a", 1)                   = 1
2572  read(3, "g", 1)                   = 1
2572  read(3, "e", 1)                   = 1
2572  read(3, ":", 1)                   = 1
2572  read(3, " ", 1)                   = 1
2572  read(3, "o", 1)                   = 1
2572  read(3, "p", 1)                   = 1
2572  read(3, "e", 1)                   = 1
2572  read(3, "n", 1)                   = 1
2572  read(3, "s", 1)                   = 1
2572  read(3, "l", 1)                   = 1
2572  read(3, "i", 1)                   = 1
2572  read(3, "d", 1)                   = 1
2572  read(3, "e", 1)                   = 1
2572  read(3, "-", 1)                   = 1
2572  read(3, "p", 1)                   = 1
2572  read(3, "y", 1)                   = 1
2572  read(3, "t", 1)                   = 1
2572  read(3, "h", 1)                   = 1
2572  read(3, "o", 1)                   = 1
2572  read(3, "n", 1)                   = 1
2572  read(3, "\n", 1)                  = 1
2572  write(4, "Package: openslide-python\n", 26) = 26
2572  read(3, "B", 1)                   = 1
2572  read(3, "i", 1)                   = 1
2572  read(3, "n", 1)                   = 1

Maybe buffering is disabled for some file descriptor where it should not
be?

-- Package-specific info:

-- (no /etc/apt/preferences present) --


-- (/etc/apt/sources.list present, but not submitted) --


-- System Information:
Debian Release: stretch/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.3.0-1-amd64 (SMP w/8 CPU cores)
Locale: LANG=de_DE.UTF-8, LC_CTYPE=de_DE.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages apt depends on:
ii  adduser                 3.113+nmu3
ii  debian-archive-keyring  2014.3
ii  gnupg                   1.4.19-6
ii  gnupg2                  2.1.10-3
ii  gpgv                    1.4.19-6
ii  libapt-pkg5.0           1.1.5
ii  libc6                   2.21-4
ii  libgcc1                 1:5.3.1-4
ii  libstdc++6              5.3.1-4

apt recommends no packages.

Versions of packages apt suggests:
pn  apt-doc     <none>
ii  aptitude    0.7.5-3
ii  dpkg-dev    1.18.3
ii  python-apt  1.1.0~beta1

-- no debconf information